Japanese Patent Unexamined Publications No. H11-288849 (Document 1) and No. 2001-297952 (Document 2) disclose methods for producing an electrode metal material which can reduce the internal resistance of a capacitor. The electrode metal material is used in contact with an electrolytic solution in a capacitor such as an EDLC. These methods reduce the internal resistance of an electrode by fixing carbon particles on a valve metal such as aluminum so as to secure the electric connection between the aluminum and the active carbon on the electrode. Japanese Patent Unexamined Publication No. 2000-269095 (Document 3) discloses a method for reducing the internal resistance of an EDLC by covering the uneven surface of a collector with carbon black particles so as to form a conductive layer. In the three methods, the collectors and the electrodes are all made of pure aluminum and pure carbon.
In Documents 1 and 2 mentioned above, the aluminum portion is covered with an oxide film caused by the water in the electrolytic solution. The potential to form the oxide film can be detected by the reaction potential on the oxidizing side, which can be measured by CV (cyclic voltammetry) or the like. An example of the CV measurement is shown in FIG. 20. In FIG. 20, the horizontal and vertical axes represent potential and current, respectively. The reference electrode is an Ag/Ag+ electrode and the counter electrode is Pt. As the working electrodes, an aluminum electrode and an aluminum electrode with carbon particles fixed thereon are used for comparison. The results show that the aluminum electrode, and the aluminum electrode with the carbon particles fixed thereon have nearly the same reaction potential. This means that both electrodes have an oxide film covering the aluminum.
These EDLCs, which have a capacitance large enough to supply a large current, can be used in electronic devices such as electric vehicles (EV) as disclosed in Japanese Patent Unexamined Publication No. H10-271611 (Document 4).
However, the aforementioned conventional EDLCs require a complicated and difficult-to-control process to from electrodes as follows. Carbon particles are fixed on aluminum and the aluminum is etched so that the carbon particles are halfway fixed on and slightly exposed from the aluminum.
The electric connection entirely depends on carbon particles, so that the reliability in fixing the carbon particles is very important for securing conduction. On the other hand, the aluminum portion covered with the oxide film caused by the water in the electrolytic solution does not contribute to conduction. Since the conductive portion (the carbon particle portion) and the nonconductive portion (the oxide film portion) are formed on the same aluminum foil surface, it is difficult to meet both conductivity and withstand voltage at the same time.
Because of being covered with the oxide film caused by the water in the electrolytic solution, the aluminum portion has a potential window whose size is limited by the reaction at the time of forming the oxide film. This results in a reduction in the withstand voltage.
In Document 4, a large number of EDLCs must be connected in series when used as the power supply unit of the EV because the withstand voltage cannot be increased. For example, if each EDLC has a withstand voltage of 2V and the EV requires a voltage of 400V, then as many as 200 EDLCs are required. This results in an increase in the size of the power supply unit. In other words, it is inevitable to improve the withstand voltage of each EDLC for the size reduction of the power supply unit. On the other hand, it is also being tried to improve the withstand voltage of electrolytic solutions, and electrolytic solutions with comparatively high withstand voltages are being developed.
However, the low withstand voltage of EDLCs results from the deterioration of the aluminum electrode foils. Therefore, deteriorated aluminum electrode foils cause a reduction in the withstand voltage of the EDLCs even with an electrolytic solution having a comparatively high withstand voltage.